Enhanced biosorption of wastewater organics using dissolved air flotation with solids recycle

ABSTRACT

Systems and methods for treating wastewater including a dissolved air flotation operation performed upon a portion of a mixed liquor output from a contact tank prior to the mixed liquor entering a biological treatment tank.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 61/819,822 titled “ENHANCED BIOSORPTIONOF WASTEWATER ORGANICS USING DISSOLVED AIR FLOTATION WITH SOLIDSRECYCLE,” filed on May 6, 2013 and under 35 U.S.C. §120 as acontinuation in part of U.S. application Ser. No. 13/210,487 titled“CONTACT-STABILIZATION/PRIME-FLOAT HYBRID,” filed on Aug. 16, 2011,which claims priority under 35 U.S.C. §119(e) to U.S. ProvisionalApplication Ser. No. 61/374,910 titled “STABILIZATION/PRIME FLOAT HYBRID(CAPTIVATOR),” filed on Aug. 18, 2010, each of which is hereinincorporated by reference in its entirety.

BACKGROUND

Aspects and embodiments of the present invention are directed towardsystems and methods for the treatment of wastewater.

SUMMARY

In accordance with an embodiment of the present invention there isprovided a method of facilitating increased operating efficiency of awastewater treatment system. The method comprises configuring adissolved air flotation (DAF) unit in a wastewater treatment system influid communication between a contact tank and a biological treatmentunit to remove solids from a portion of a first mixed liquor output fromthe contact tank prior to the portion of the first mixed liquor enteringthe biological treatment unit and to recycle at least a portion of thesolids to the contact tank, the recycle of the at least a portion of thesolids to the contact tank reducing an amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of recycling the atleast a portion of the solids to the contact tank.

In some embodiments, greater than 50% of the solids are recycled fromthe DAF unit to the contact tank.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to increase biogasproduction of an anaerobic digester of the wastewater treatment systemhaving an inlet in fluid communication with an outlet of the DAF unit,at least a second portion of the solids removed in the DAF unit beingdirected into the anaerobic digester.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to reduce the energyconsumption of the wastewater treatment system.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet, a second inlet, and anoutlet and a dissolved air flotation tank having an inlet in fluidcommunication with the outlet of the contact tank, a first outlet, and asecond outlet. The wastewater treatment system further comprises anaerated anoxic tank having a first inlet in fluid communication with theoutlet of the contact tank, a second inlet, and an outlet and aerobictank having a first inlet in fluid communication with the outlet of theaerated anoxic tank, a second inlet in fluid communication with thefirst outlet of the dissolved air flotation tank, and an outlet. Thewastewater treatment system further comprises a clarifier having aninlet in fluid communication with the outlet of the aerobic tank and anoutlet in fluid communication with the second inlet of the contact tankand with the second inlet of the aerated anoxic tank.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank, mixing the wastewaterwith activated sludge in the contact tank to form a mixed liquor,transporting a first portion of the mixed liquor to a dissolved airflotation tank, separating the first portion of the mixed liquor in thedissolved air flotation tank to form a dissolved air flotation tankeffluent and waste biosolids, transporting a second portion of the mixedliquor to an aerated anoxic treatment tank, biologically treating thesecond portion of the mixed liquor in the aerated anoxic treatment tankto form an anoxic mixed liquor, transporting the anoxic mixed liquor toan aerobic treatment tank, transporting the dissolved air flotation tankeffluent to the aerobic treatment tank, biologically treating the anoxicmixed liquor and the dissolved air flotation tank effluent in theaerobic treatment tank to form an aerobic mixed liquor, transporting theaerobic mixed liquor to a clarifier, separating the aerobic mixed liquorin the clarifier to form a clarified effluent and a return activatedsludge, recycling a first portion of the return activated sludge to thecontact tank, and recycling a second portion of the return activatedsludge to the aerated anoxic treatment tank.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet configured to receivewastewater to be treated, a second inlet, and an outlet. The contacttank is configured to mix the wastewater to be treated with activatedsludge to form a first mixed liquor. The system further comprises a DAFunit having an inlet in fluid communication with the outlet of thecontact tank, a solids outlet, a DAF unit effluent outlet, and a gasinlet. The gas inlet is configured to introduce gas into the DAF unit tofacilitate the flotation of suspended matter from the first mixed liquorand the removal of the suspended matter from the DAF unit. The solidsoutlet is in fluid communication with the first inlet of the contacttank and configured to transfer at least a portion of the suspendedmatter from the DAF unit to the first inlet of the contact tank. Thesystem further comprises a biological treatment unit having a firstinlet in fluid communication with the outlet of the contact tank, asecond inlet, a third inlet in fluid communication with the DAF uniteffluent outlet, and an outlet. The biological treatment unit isconfigured to biologically break down organic components of the firstmixed liquor and of an effluent from the DAF unit to form a second mixedliquor. The system further comprises a clarifier having an inlet influid communication with the outlet of the biological treatment unit, aneffluent outlet, and a return activated sludge outlet in fluidcommunication with the second inlet of the contact tank and with thesecond inlet of the biological treatment unit. The clarifier isconfigured to output a clarified effluent through the effluent outletand a return activated sludge though the return activated sludge outlet.

In accordance with some aspects of the wastewater treatment system, thebiological treatment unit includes an aerated anoxic region having afirst inlet in fluid communication with the outlet of the contact tank,a second inlet, and an outlet and an aerobic region having a first inletin fluid communication with the outlet of the aerated anoxic region, asecond inlet in fluid communication with the DAF unit effluent outlet,and an outlet.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are included in a sametreatment tank.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are separated by apartition.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region is included in a first treatment tank and theaerobic region is included in a second treatment tank distinct from thefirst treatment tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system comprises a first sub-system including thecontact tank and the DAF unit which is physically separated from asecond sub-system including the biological treatment unit and theclarifier.

In accordance with some aspects of the wastewater treatment system, thecontact tank and the aerated anoxic region are included in a same tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises an anaerobic digesterhaving an inlet in fluid communication with the solids outlet of the DAFunit and an outlet.

In accordance with some aspects of the wastewater treatment system, theoutlet of the anaerobic digester is in fluid communication with at leastone of the contact tank and the biological treatment unit.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a primary clarifier havingan inlet in fluid communication with a source of the wastewater to betreated and a solids-lean outlet in fluid communication with the contacttank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a thickener having aninlet in fluid communication with a solids-rich outlet of the primaryclarifier and an outlet in fluid communication with the anaerobicdigester.

In accordance with some aspects of the wastewater treatment system, theprimary clarifier further comprises a solids-rich outlet in fluidcommunication with the DAF unit.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank including an activatedsludge, mixing the wastewater with activated sludge in the contact tankto form a mixed liquor, and directing a first portion of the mixedliquor to a DAF unit. The method further comprises separating the firstportion of the mixed liquor in the DAF unit to form a DAF unit effluentand separated biosolids, directing at least a portion of the separatedbiosolids from the DAF unit to the contact tank, directing a secondportion of the mixed liquor to a biological treatment unit, directingthe DAF unit effluent to the biological treatment unit, biologicallytreating the mixed liquor and the DAF unit effluent in the biologicaltreatment unit to form a biologically treated mixed liquor, anddirecting the biologically treated mixed liquor to a clarifier. Themethod further comprises separating the biologically treated mixedliquor in the clarifier to form a clarified effluent and a returnactivated sludge, recycling a first portion of the return activatedsludge to the contact tank, recycling a second portion of the returnactivated sludge to the biological treatment unit, and directing theclarified effluent to a treated wastewater outlet.

In accordance with some aspects of the method of treating wastewaterwherein the biological treatment unit includes an aerated anoxictreatment unit and an aerobic treatment unit, the method furthercomprises directing the second portion of the mixed liquor to theaerated anoxic treatment unit, treating the second portion of the mixedliquor in the aerated anoxic treatment unit to form an anoxic mixedliquor, directing the anoxic mixed liquor to the aerobic treatment unit,directing the DAF unit effluent to the aerobic treatment unit, treatingthe anoxic mixed liquor and the DAF unit effluent in the aerobictreatment tank to form an aerobic mixed liquor, directing the aerobicmixed liquor to the clarifier, separating the aerobic mixed liquor inthe clarifier to form the clarified effluent and the return activatedsludge, and recycling the second portion of the return activated sludgeto the aerated anoxic treatment unit.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge and the second portionof the return activated sludge comprise about 100% of all returnactivated sludge formed in the clarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge comprises between about10% and about 20% of all return activated sludge recycled from theclarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the mixed liquor comprises between about one thirdand about two thirds of all mixed liquor formed in the contact tank.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 60% and about 100% of suspendedsolids in the first portion of the mixed liquor from the first portionof the mixed liquor.

In accordance with some aspects of the method of treating wastewater, anamount of suspended solids removed in the DAF unit is adjusted basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 40% and about 80% of biologicaloxygen demand in the first portion of the mixed liquor from the firstportion of the mixed liquor.

In accordance with some aspects of the method of treating wastewater,the method further comprises treating at least a portion of the wastebiosolids in an anaerobic digester to produce an anaerobically digestedsludge.

In accordance with some aspects of the method of treating wastewater,the method further comprises recycling at least a portion of theanaerobically digested sludge to at least one of the contact tank andthe biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the method further comprises separating the water to be treated into asolids-lean portion and a solids-rich portion, directing the solids-richportion into a thickener to produce a solids-rich output and asolids-lean effluent, directing the solids-lean portion into the contacttank, directing the solids-rich output from the thickener into theanaerobic digester, and directing the solids-lean effluent of thethickener into the contact tank.

In accordance with another embodiment of the present invention there isprovided method of facilitating increased operating efficiency of awastewater treatment system. The method comprises providing a DAF unitin a wastewater treatment system in fluid communication between acontact tank and a biological treatment unit, the DAF unit configured toremove solids from a portion of a first mixed liquor output from thecontact tank prior to the portion of the first mixed liquor entering thebiological treatment unit and to recycle at least a portion of thesolids to the contact tank, reducing the amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of the DAF unit,and providing for a solids-liquid separation unit in fluid communicationdownstream of the biological treatment unit to recycle a returnactivated sludge formed from a mixed liquor output from the biologicaltreatment unit to the contact tank.

In accordance with some aspects, the method further comprises providingfor between about 10% and about 20% of the return activated sludgeformed to be recycled to the contact tank.

In accordance with some aspects, the method further comprises adjustingan amount of return activated sludge recycled to the contact tank basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects, the method further comprises providingan anaerobic digester having an inlet in fluid communication with anoutlet of the DAF unit and an outlet in fluid communication with atleast one of an inlet of the contact tank and an inlet of the biologicaltreatment unit.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a block flow diagram of a wastewater treatment system inaccordance with an embodiment of the present invention;

FIG. 2 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 3 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 4 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 5 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 6 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 7 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 8 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 9 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 10 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment of the present invention;

FIG. 11 illustrates a first set of results of a test of a system inaccordance with an embodiment of the present invention; and

FIG. 12 illustrates a second set of results of a test of a system inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

As the term is used herein, an “upstream” unit operation refers to afirst unit operation which is performed upon a fluid undergoingtreatment prior to a second unit operation. Similarly, an “upstream”treatment vessel or portion thereof refers to a first treatment vesselor portion thereof in which a first unit operation is performed prior toa second unit operation performed in a second treatment vessel orportion thereof. A “downstream” unit operation refers to a second unitoperation which is performed upon a fluid undergoing treatmentsubsequent to a first unit operation. Similarly, a “downstream”treatment vessel or portion thereof refers to a second treatment vesselor portion thereof in which a second unit operation is performedsubsequent to a first unit operation performed in a first treatmentvessel or portion thereof. An upstream unit operation and/or treatmentvessel having an outlet in “direct fluid communication” with an inlet ofa downstream unit operation and/or treatment vessel directs materialoutput from the outlet of the upstream unit operation and/or treatmentvessel into the inlet of the downstream unit operation and/or treatmentvessel without any intervening operations performed on the material. Afirst unit operation and/or treatment vessel described herein as beingin fluid communication with a second unit operation and/or treatmentvessel should be understood as being in direct fluid communication withthe second unit operation and/or treatment vessel unless explicitlydescribed as otherwise. Conduits which provide fluid communicationbetween a first and a second unit operation and/or treatment vessel areto be understood as providing direct fluid communication between thefirst and second unit operation and/or treatment vessel unlessexplicitly described as otherwise.

Various unit operations and/or treatment vessels disclosed hereinseparate fluid and/or sludge into a solids-rich portion and asolids-lean portion wherein the solid-lean potion has a lowerconcentration of solids than the solids-rich portion. As the term isused herein, an “effluent” of a unit operation and/or treatment vesselrefers to the solids-lean portion of the separated fluid and/or sludge.“Recycle” of material refers to directing material from an outlet of adownstream unit operation and/or treatment vessel to an inlet of a unitoperation and/or treatment vessel upstream of the downstream unitoperation and/or treatment vessel.

Co-pending U.S. application Ser. No. 13/210,487, titled “ContactStabilization/Prime Float Hybrid” is incorporated herein by reference inits entirety for all purposes.

Aspects and embodiments of the present invention are directed towardsystems and methods for treating wastewater. As used herein the term“wastewater” includes, for example, municipal wastewater, industrialwastewater, agricultural wastewater, and any other form of liquid to betreated containing undesired contaminants. Aspects and embodiments ofthe present invention may be utilized for primary wastewater treatment,secondary wastewater treatment, or both. Aspects and embodiments of thepresent invention may remove sufficient contaminants from wastewater toproduce product water that may be used for, for example, irrigationwater, potable water, cooling water, boiler tank water, or for otherpurposes.

In some embodiments, the apparatus and methods disclosed herein provideadvantages with regard to, for example, capital costs, operationalcosts, and environmental-friendliness as compared to conventionalbiological wastewater treatment systems. In some embodiments a dissolvedair flotation system is included in a main stream of wastewater enteringa biological wastewater treatment system. The dissolved air floatationsystem may remove a significant amount of biological oxygen demand, forexample, particulate biological oxygen demand, from wastewater prior tothe wastewater entering the biological treatment portion of thewastewater treatment system. This provides for a reduction in the sizeof the biological treatment portion of the wastewater treatment systemfor a given wastewater stream as compared to a conventional wastewatertreatment system and a commensurate reduced capital cost for the overallsystem. Utilization of the dissolved air flotation system also reducesthe requirement for aeration in the biological treatment portion of thetreatment system to effect oxidation of the biological oxygen demand ofthe wastewater, reducing operating costs. The amount of waste sludgegenerated by the biological treatment portion of the treatment system isalso reduced, reducing the amount of waste which would need to bedisposed of or otherwise further treated. The material removed from thewastewater in the dissolved air flotation system may be utilized toproduce energy, for example, in the form of biogas in a downstreamanaerobic digestion system. The biogas may be used to provide salableenergy through combustion or through use in, for example, fuel cells.

A first embodiment, indicated generally at 100, is illustrated inFIG. 1. Wastewater from a source of wastewater 105 is directed into acontact tank 110 through an inlet of the contact tank. In the contacttank 110, the wastewater is mixed with activated sludge recycled througha conduit 175 from a downstream biological treatment process describedbelow. In some embodiments, the contact tank 110 is aerated tofacilitate mixing of the wastewater and the activated sludge. Theaeration gas may be an oxygen containing gas, for example, air. Thecontact tank 110 may be provided with sufficient oxygen such thataerobic conditions are maintained in at least a portion of the contacttank 110. For example, the contact tank 110 may be aerated. Suspendedand dissolved solids in the wastewater, including oxidizable biologicalmaterials (referred to herein as Biological Oxygen Demand, or BOD), areabsorbed into the activated sludge in the contact tank, forming a firstmixed liquor. A portion of the BOD may also be oxidized in the contacttank 110. The residence time of the wastewater in the contact tank maybe sufficient for the majority of the BOD to be absorbed by theactivated sludge, but no so long as for a significant amount ofoxidation of the BOD to occur. In some embodiments, for example, lessthan about 10% of the BOD entering the contact tank 110 is oxidized inthe contact tank. The residence time of the wastewater in the contacttank is in some embodiments from about 30 minutes to about two hours,and in some embodiments, from about 45 minutes to about one hour. Theresidence time may be adjusted depending upon factors such as the BOD ofthe influent wastewater. A wastewater with a higher BOD may requirelonger treatment in the contact tank 110 than wastewater with a lowerBOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a dissolved air flotation (DAF) system 120 through conduit114. The DAF system may include a vessel, tank, or other open or closedcontainment unit configured to perform a dissolved air flotationoperation as described below. For the sake of simplicity a dissolved airflotation system will be referred to herein as a “DAF unit.” The DAFunit 120 may function as both a thickener and a clarifier. FIG. 1illustrates two DAF units 120 operating in parallel, however, otherembodiments may have a single DAF unit or more than two DAF units.Providing multiple DAF units provides for the system to continueoperation if one of the DAF units is taken out of service for cleaningor maintenance.

Before entering the DAF unit(s), air or another gas may be dissolved inthe first mixed liquor under pressure. The pressure may be released asthe first mixed liquor enters the DAF unit(s) 120, resulting in the gascoming out of solution and creating bubbles in the mixed liquor. In someembodiments, instead of dissolving gas into the first mixed liquor, afluid, for example, water having a gas, for example, air, dissolvedtherein, is introduced into the DAF unit(s) 120 with the first mixedliquor. Upon the mixing of the first mixed liquor and the gas-containingfluid, bubbles are produced. The bubbles formed in the DAF unit(s) 120adhere to suspended matter in the first mixed liquor, causing thesuspended matter to float to the surface of the liquid in the DAFunit(s) 120, where it may be removed by, for example, a skimmer.

In some embodiments, the first mixed liquor is dosed with a coagulant,for example, ferric chloride or aluminum sulfate prior to or afterintroduction into the DAF unit(s) 120. The coagulant facilitatesflocculation of suspended matter in the first mixed liquor.

In the DAF unit(s) 120 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process. At least a portion of any oil that maybe present in the first mixed liquor may also be removed in the DAFunit(s) 120. In some embodiments, a majority, for example, about 60% ormore, about 75% or more, or about 90% or more of the suspended solids inthe first mixed liquor introduced into the DAF unit(s) 120 is removedand about 40% or more, for example, about 50% or more or about 75% ormore of the BOD is removed. Removal of the BOD may include enmeshmentand adsorption in the first mixed liquor and/or oxidation of the BOD andthe formation of reaction products such as carbon dioxide and water. Inother embodiments, up to about 100% of the suspended solids is removedin the DAF unit(s) 120 and a majority, for example, up to about 80% ofthe BOD is removed.

In some embodiments, suspended solids removed in the DAF unit(s) 120 aresent out of the system as waste solids through a conduit 125. Thesewaste solids may be disposed of, or in some embodiments, may be treatedin a downstream process, for example, an anaerobic digestion process oranaerobic membrane bioreactor to produce useful products, for example,biogas and/or usable product water.

In other embodiments, at least a portion of the suspended solids removedin the DAF unit(s) 120 are recycled back to the contact tank 110 throughconduits 125 and 126. Conduit 126 may branch off of conduit 125 asillustrated, or may be connected to a third outlet of the DAF unit(s)120, in which case suspended solids removed in the DAF unit(s) 120 arerecycled back to the contact tank 110 through conduit 126 only. Theamount of solids recycled from DAF unit(s) 120 to the contact tank 110may range from about 1% to about 100% of a total amount of solidsremoved from the first mixed liquor in the DAF unit(s) 120. The amountof solids recycled from DAF unit(s) 120 to the contact tank 110 may be amajority of a total amount of solids removed from the first mixed liquorin the DAF unit(s) 120, for example, greater than about 50%, betweenabout 50% and about 95%, or between about 60% and about 80% of the totalamount of solids removed from the first mixed liquor in the DAF unit(s)120.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110is counter to the conventional operation of wastewater treatment systemsincluding DAF units. Typically, DAF units are utilized in wastewatertreatment systems to remove solids from the wastewater, thus reducingthe need for biological treatment of these removed solids and reducingthe energy requirements of the wastewater treatment system by, forexample, reducing the amount of air needed to be supplied to an aeratedbiological treatment vessel to oxidize the removed solids. It is counterto conventional operation of wastewater treatment systems tore-introduce floated solids separated from mixed liquor from a contacttank in DAF unit(s) back to the contact tank. Typically, after solidsare separated from mixed liquor from a contact tank in DAF unit(s),reintroducing the separated solids into mixed liquor in the contact tankand force the solids to go through the same separation process in theDAF unit(s) again reducing the efficiency of the system. Such a solidsrecycle from DAF unit(s) to a contact tank directly upstream of the DAFunit(s) would cause a need for a greater amount of contact tank capacityand a greater amount of DAF unit capacity. Such a solids recycle fromDAF unit(s) to a contact tank directly upstream of the DAF unit(s) wouldalso require more air flow to the DAF unit(s) to remove the recycledsolids from the mixed liquor in addition to any solids that would bepresent in the absence of the solids recycle. It has been discovered,however, that benefits may be achieved by the counterintuitivere-introduction of solids removed in DAF unit(s) back into the contacttank of a wastewater treatment system from which mixed liquor issupplied to the DAF unit(s).

For example, by recycling the solids removed by the DAF unit(s) 120 tothe contact tank 110, the amount of total suspended solids (TSS) in thecontact tank 110 may be increased as compared to methods not including arecycle of solids from the DAF unit(s) 120 to the contact tank 110. Theincreased TSS level in the contact tank 110 may provide for additionalsoluble BOD to be adsorbed in the contact tank 110 as compared to acontact tank 110 having a lower level of TSS. In some embodiments, adesirable TSS level in the contact tank 110 may be between about 1,200mg/L and about 3,500 mg/L.

The removal of the additional soluble BOD in the contact tank 110 due tothe higher TSS level in the contact tank 110, resulting from the recycleof solids from the DAF unit(s) 120 to the contact tank 110, provides forthe removal of this additional BOD as solids in the DAF unit(s) 120. Theadditional BOD removed as solids in the DAF unit(s) 120 may be directedto an anaerobic digester (for example, anaerobic digester 490illustrated in FIG. 4) rather than an aerated biological treatment unit(for example, biological treatment unit 130), thus reducing the need foraeration power in the biological treatment unit and increasing theamount of biogas that could be produced in the anaerobic digester.

When supplied with recycled solids from the DAF unit(s) 120, the contacttank 110 may have a hydraulic retention time (HRT) of between about 15minutes and about one hour and a solids retention time (SRT) of betweenabout 0.5 days and about two days to effectively adsorb soluble BOD. Inother embodiments, the SRT in the contact tank may be between about 0.2and about 0.4 days. When the contact tank 110 includes TSS in a range ofbetween about 1,200 mg/L and about 3,500 mg/L, a sludge age (SRT) in thecontact tank may range from about one to about two days.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110provides for the contact tank 110 to function as a high rate activatedsludge system while the DAF unit(s) 120 function a solids-liquidseparator. Recycling solids removed in the DAF unit(s) 120 to thecontact tank 110 provides for greater oxidation of BOD in the contacttank 110 than in systems where solids removed from the DAF unit(s) 120are not recycled to the contact tank because the solids recycled to thecontact tank includes living bacteria capable of oxidizing BOD. Forexample, in systems and methods where solids removed in the DAF unit(s)120 are recycled to the contact tank 110, oxidation of greater thanabout 10% of the BOD in wastewater influent to the contact tank 110 maybe oxidized in the contact tank 110. Recycling solids removed in the DAFunit(s) 120 to the contact tank 110 may thus reduce the amount of BODthat needs to be treated in downstream unit operations, for example, inthe biological treatment unit 130 discussed below, thus reducing thepower requirements for the downstream unit operations. The SRT of thecontact tank 110 may be adjusted to optimize BOD removal of particulate,colloidal, and soluble BOD fractions.

Effluent from the DAF unit(s) 120 is directed through conduit 124 intothe biological treatment unit 130, which may include one or moretreatment tanks. In some embodiments, the biological treatment unit 130may comprise a contact stabilization vessel. A portion of the effluentmay be recycled (recycle system not shown in FIG. 1) to supply gasbubbles to the DAF unit(s) 120. A gas may be dissolved into the recycledportion of effluent, which is then directed back into the DAF unit(s)120 and mixed with influent first mixed liquor.

A second portion of the first mixed liquor formed in the contact tank isdirected into the biological treatment unit 130 through a conduit 115.In some embodiments, about a half of the first mixed liquor formed inthe contact tank is directed into the DAF unit(s) 120 and about a halfof the first mixed liquor formed in the contact tank is directed throughthe conduit 115 into the biological treatment unit 130. In otherembodiments, between about one third and two thirds of the first mixedliquor formed in the contact tank is directed into the DAF unit(s) 120and the remainder of the first mixed liquor formed in the contact tankis directed through the conduit 115 into the biological treatment unit130. The amount of the first mixed liquor directed into the DAF unit(s)120 as opposed to the biological treatment unit 130 may be varied basedupon such factors as the concentration of the first mixed liquor and theeffectiveness of the first mixed liquor at enmeshing BOD in the contacttank 110.

For example, if it was desired to remove a greater rather than a lesseramount of solids in the DAF unit(s) 120, a greater fraction of the firstmixed liquor from the contact tank would be directed to the DAF unit(s)120 when the first mixed liquor had a lower rather than a higherconcentration of solids. Similarly, if it was desired to remove agreater rather than a lesser amount of BOD in the DAF unit(s) 120, agreater fraction of the first mixed liquor from the contact tank wouldbe directed to the DAF unit(s) 120 when the first mixed liquor had alesser rather than a greater effectiveness at enmeshing BOD in thecontact tank.

In the biological treatment unit 130, the effluent from the DAF unit(s)120 and the first mixed liquor formed in the contact tank 110 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 130 includes oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 130 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 130 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 130. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor, and the biologicaltreatment unit 130, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 130. The residence time of the second mixedliquor in the biological treatment unit 130 may be sufficient to oxidizesubstantially all BOD in the second mixed liquor. Residence time for thesecond mixed liquid in the biological treatment unit 130 may be fromabout three to about eight hours. This residence time may be increasedif the influent wastewater to be treated and/or the second mixed liquorcontains a high level of BOD or decreased if the influent wastewater tobe treated and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 130is directed through a conduit 135 into a separation apparatus, which mayinclude, for example, a clarifier 140, a gravity separation apparatus,and/or another form of separation apparatus. Effluent from the clarifier140 may be directed to a product water outlet through a conduit 145 orbe sent on for further treatment. Activated sludge separated fromeffluent in the clarifier may be recycled back upstream to a wastewaterinlet of the system, the source of wastewater, the contact tank 110through conduits 155 and 175, and/or the biological treatment unit 130through conduits 155 and 165. In some embodiments 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments between about 10% and about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through the conduit175 and between about 80% and 90% of the recycled sludge is directedinto the biological treatment unit 130 through the conduit 165. Theamount of recycled sludge directed to the wastewater inlet and contacttank through the conduit 175 may be set at a higher end of this rangewhen the incoming wastewater has a high level of BOD and/or when therecycled sludge is less rather than more effective at enmeshing BOD inthe contact tank 110. The amount of recycled sludge directed to thewastewater inlet and contact tank through the conduit 175 may be set ata lower end of this range when the incoming wastewater has a low levelof BOD and/or when the recycled sludge is more rather than lesseffective at enmeshing BOD in the contact tank 110.

The amount of activated sludge separated in the clarifier 140 which isrecycled to the contact tank 110 and/or biological treatment unit 130may also be adjusted based on a fraction of the first mixed liquor fromthe contact tank 110 which is directed to the DAF unit(s) 120, theamount of activated sludge which is removed in the DAF units(s) 120,and/or the amount of activated sludge removed in the DAF units(s) 120which is recycled to the contact tank 110. The amount of activatedsludge which is recycled to the contact tank 110 and/or biologicaltreatment unit 130 may be an amount equal to or greater than an amountrequired to maintain a desired population of bacteria in the biologicaltreatment unit 130 to perform biological treatment of the second mixedliquor within a desired timeframe and/or to protect against depletion ofthe bacterial population in the event of temporary disruptions in theoperation of the treatment system. For example, the amounts of activatedsludge which is recycled to the contact tank 110 or biological treatmentunit 130 may be set such that sufficient bacteria containing solids arepresent in the biological treatment unit 130 to result in a SRT ofbetween about one and about 10 days in the biological treatment unit130. Similarly, an amount or fraction of the first mixed liquor directedinto the DAF unit(s) 120 may be adjusted based on the amount ofactivated sludge recycled from the clarifier 140, the efficiency ofremoval of solids in the DAF unit(s) 120 and/or the concentration of oneor more types of bacteria in the biological treatment unit 130 to, forexample, establish or maintain a desired population of bacteria in thebiological treatment unit 130.

In the embodiment illustrated in FIG. 1, and in the additionalembodiments described below, it should be understood that the variousconduits illustrated may be provided with, for example, pumps, valves,sensors, and control systems as needed to control the flow of fluidstherethrough. These control elements are not illustrated in the figuresfor the sake of simplicity.

In another embodiment, indicated generally at 200 in FIG. 2, thebiological treatment unit 130 includes an aerobic region 150 and anaerated anoxic region 160. The aerobic region 150 is in fluidcommunication downstream of the aerated anoxic region 160 and receivesbiologically treated anoxic mixed liquor from the aerated anoxic region.In some embodiments, the aerobic region 150 may be formed in a samevessel or tank as the aerated anoxic region 160 and separated therefromby a partition or weir 195. In other embodiments, the aerobic region 150may be physically separate from the aerated anoxic region 160. Forexample, the aerobic region 150 and the aerated anoxic region 160 mayoccupy distinct vessels or tanks or may be otherwise separated from oneanother. In further embodiments the contact tank 110 may be combinedwith the aerated anoxic region 160 in the same tank.

In the system of FIG. 2 effluent from the DAF unit(s) 120 is directedinto the aerobic region 150 without first passing through the aeratedanoxic region 160. In other embodiments, the effluent from the DAFunit(s) 120 may be introduced into the aerated anoxic region 160 andthen directed into the aerobic region 150.

Another embodiment, indicated generally at 300, is illustrated in FIG.3. In this embodiment, the wastewater treatment system 300 is brokeninto two separate but interconnected subsystems, one subsystem 300Aincluding a contact tank 210 and DAF unit(s) 220, and a second subsystem300B including a biological treatment unit 230 and a separationapparatus 240. In the first subsystem 300A influent wastewater from asource of wastewater 205A is directed into the contact tank 210. In thecontact tank, the wastewater is mixed with activated sludge recycledthrough a conduit 275 from a biological treatment process included insubsystem 300B described below. In some embodiments, the contact tank210 is aerated to facilitate mixing of the wastewater and the activatedsludge. Suspended and dissolved solids in the wastewater areadsorbed/absorbed into the activated sludge in the contact tank 210,forming a first mixed liquor. A portion of the BOD in the influentwastewater may be oxidized in the contact tank 210. The residence timeof the wastewater in the contact tank may be sufficient for the majorityof the BOD to be adsorbed/absorbed by the activated sludge, but no solong as for a significant amount of oxidation of the BOD to occur. Insome embodiments, for example, less than about 10% of the BOD enteringthe contact tank 210 is oxidized in the contact tank. The residence timeof the wastewater in the contact tank is in some embodiments from about30 minutes to about two hours, and in some embodiments, from about 45minutes to about one hour. The residence time may be adjusted dependingupon factors such as the BOD of the influent wastewater. A wastewaterwith a higher BOD may require longer treatment in the contact tank 210than wastewater with a lower BOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a DAF unit 220 through conduit 214. FIG. 3 illustrated twoDAF units 220 operating in parallel, however other embodiments may havea single DAF unit or more than two DAF units. Providing multiple DAFunits provides for the system to continue operation if one of the DAFunits is taken out of service for cleaning or maintenance. A secondportion of the first mixed liquor formed in the contact tank is directedinto the biological treatment unit 230 in the second subsystem 300Bthrough a conduit 215. In some embodiments, about a half of the firstmixed liquor formed in the contact tank is directed into the DAF unit(s)220 and about a half of the first mixed liquor formed in the contacttank is directed through the conduit 215 into the biological treatmentunit 230. In other embodiments, between about one third and two thirdsof the first mixed liquor formed in the contact tank is directed intothe DAF unit(s) 220 and the remainder of the first mixed liquor formedin the contact tank is directed through the conduit 215 into thebiological treatment unit 230. The amount of the first mixed liquordirected into the DAF unit(s) 220 as opposed to the biological treatmentunit 230 may be varied based upon such factors as the concentration ofthe first mixed liquor and the effectiveness of the first mixed liquorat enmeshing BOD in the contact tank 210.

In the DAF unit(s) 220 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process such as that described above withreference to DAF unit(s) 120. The removed suspended solids may be sentout of the system as waste solids through a waste conduit 225. Thesewaste solids may be disposed of or treated in a downstream process, forexample, an anaerobic digestion process or anaerobic membrane bioreactorto produce biogas and/or usable product water. Effluent from the DAFunit(s) 220 is directed to an outlet 235 from which it may be used asproduct water or sent on for further treatment.

In some embodiments, a portion of the suspended solids removed from thefirst mixed liquor in the DAF unit(s) 220 may be recycled to the contacttank 210 through conduits 225 and 226 in a similar manner as the recycleof suspended solids removed in the DAF unit(s) 120 to the contact tank110 described above with reference to FIG. 1.

In the second subsystem 300B, influent wastewater from a source ofwastewater 205B is introduced into the biological treatment unit 230.The source of wastewater 205B may be the same as or different from thesource of wastewater 205A. In the biological treatment unit 230 thewastewater and the first mixed liquor formed in the contact tank 210 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 230 may include oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 230 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 230 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 230. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor and the biologicaltreatment unit 230, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 230.

Residence time for the second mixed liquid in the biological treatmenttank 230 may be from about three to about eight hours. This residencetime may be increased if the influent wastewater to be treated and/orthe second mixed liquor contains a high level of BOD or decreased if thewastewater and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 230is directed through a conduit 235 into a separation apparatus, which mayinclude, for example, a clarifier 240. Effluent from the clarifier 240may be directed to a product water outlet through a conduit 245 or besent on for further treatment. Activated sludge separated from effluentin the clarifier may be recycled back upstream to the biologicaltreatment unit 230 and/or to the contact tank 210 in subsystem 300Athrough a conduit 255. In some embodiments about 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments from about 10% to about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through a conduit 275and from about 80% to about 90% of the recycled sludge is directed intothe biological treatment unit 230 through a conduit 265.

Utilizing DAF units as described above in a wastewater treatment systemprovides several advantages over similar wastewater treatment systemsoperated without DAF units. Because the DAF units remove a significantportion of suspended solids from influent wastewater without the needfor oxidation of these solids, the size of other components of thesystem may be reduced, resulting in a lower capital cost for the system.For example, primary clarifiers may be omitted from the wastewatertreatment system. Due to the reduced amount of oxidized solids to beremoved from the system, a final clarifier, such as the clarifier 140,may be reduced in size, in some embodiments by about 50%. Because alower amount of BOD enters the biological treatment unit (for example,the biological treatment unit 130), the size of the biological treatmentunit may be reduced, in some embodiments by about 30%. There is also alesser requirement for oxygen in the biological treatment unit whichallows for the capacity and power requirements of an aeration system inthe biological treatment unit to also be reduced, in some embodiments byabout 30%. The reduced size of the components of the treatment systemprovides for a decreased footprint of the system. For example, awastewater treatment plant with a capacity to treat 35 million gallonsper day (MGD) of wastewater with an influent BOD of 200 mg/L wouldrequire about 150,000 ft² of treatment units with a conventional designapproach; with embodiments of the present invention the footprint couldbe reduced to about 75,000 ft².

In other embodiments of systems and methods in accordance with thepresent invention, a wastewater treatment system, such as any of thosedescribed above, may further include an anaerobic treatment unit (ananaerobic digester). Non-limiting examples of components or portions ofanaerobic systems that can be utilized in one or more configurations ofthe wastewater treatment systems include, but are not limited to, theDYSTOR® digester gas holder system, the CROWN® disintegration system,the PEARTH® digester gas mixing system, the PFT® spiral guided digestergas holder, the PFT® vertical guided digester holder, the DUO-DECK™floating digester cover, and the PFT® heater and heat exchanger system,from Evoqua Water Technologies.

The anaerobic digester may be utilized to treat mixed liquor, which mayinclude suspended solids, sludge, and/or solids-rich or solids-leanfluid streams, from one or more other treatment units of the wastewatertreatment system. At least a portion of an anaerobically treated sludgeproduced in the anaerobic digester may be recycled back to one or moreother treatment units of the wastewater treatment system. The nature andfunction of the anaerobic digester and associated recycle streams may besimilar to those described in co-pending U.S. patent application Ser.No. 13/034,269, titled “Hybrid aerobic and anaerobic wastewater andsludge treatment systems and methods,” published as US 2011/0203992 A1,which is herein incorporated by reference in its entirety for allpurposes.

The systems and components of embodiments of the invention may providecost advantages relative to other wastewater treatment systems throughthe use of biological treatment processes in combination with anaerobicdigestion. The wastewater treatment systems and processes of embodimentsof the present invention can reduce sludge production through the use ofvarious unit operations including aerobic and anaerobic biologicalprocesses and recycle streams. The wastewater treatment processes alsoovercome some of the technical difficulties associated with use of someanaerobic wastewater treatment processes, by, for example, concentratingor strengthening the sludge introduced into the anaerobic digester.Additionally, costs associated with use of a conventional aerobicstabilization unit are typically reduced because less aeration wouldtypically be required in the aerobic processes due to the use of theanaerobic digester and various recycle streams. The various processescan also generate methane as a product of the anaerobic digestionprocess, which can be used as an energy source. In certain embodiments,a large portion of the chemical oxygen demand (COD) and BOD present ininfluent wastewater to be treated can be reduced using the anaerobicdigester. This can reduce the aeration and oxygen requirements, andthus, operation costs of the wastewater treatment system, and increasethe amount of methane produced that can be used as an energy source.Additionally, because anaerobic digestion can be used to reduce COD andBOD in the sludge, the sludge yield can also be reduced. The reductionof COD and/or BOD in the anaerobic treatment unit may also provide for areduction in size of the stabilization tank or other aerobic treatmentunit in the wastewater treatment system as compared to systems notutilizing the anaerobic digester.

Embodiments of the present invention may provide for the recirculationof aerobic bacteria, anaerobic bacteria, or both through various unitoperations of the treatment system.

It was previously believed that methanogens were strict anaerobicbacteria that would die quickly in an aerobic environment. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of methanogenic organisms. One advantageousfeature of the treatment systems of the present application involvesproviding a large amount of methanogens through the anaerobic recycle toa contact stabilization process through the unique internal anaerobicsludge recycle path. At least a portion of the methanogenic bacteriareturn to the anaerobic digester, thereby seeding the anaerobic digesterwith methanogenic bacteria to join the existing population of the viablemethanogens in the anaerobic digester. This reduces the need for theanaerobic digester to have a size and resultant hydraulic residence timeor solids retention time to maintain a stable methanogenic bacteriapopulation in the absence of bacterial seeding, as in previously knownprocesses.

The concentration of seeding methanogenic bacteria, on a basis of acount of microorganisms, provided at the input of the anaerobic digestermay in some embodiments be at least a target percentage, such as about10% or more, of the concentration of the methanogenic bacteria presentin the anaerobically digested sludge stream exiting the anaerobicdigester. In some embodiments, this percentage may be, for example,about 25% or more, about 33% or more, about 50% or more, or about 75% ormore.

The anaerobic digester of systems in accordance with the presentinvention may be sized smaller than those in previously known systems.The methanogenic bacterial seeding of the anaerobic digester alsoprovides for a safety factor against disruptions of the anaerobicdigestion process. In the event of anaerobic digestion process upset orfailure, the anaerobic digesters of the presently disclosed systemswould recover faster than that the anaerobic digesters in previouslyknown systems because the seeding of the anaerobic digester withmethanogenic bacteria would add to the rate of replenishment ofmethanogenic bacteria in the anaerobic reactor due to the growth ofthese bacteria therein, reducing the time required for the anaerobicdigester to achieve a desired concentration of methanogenic bacteria.

The advantage of methanogen recycle can be estimated as follow:

$\theta_{x} = \frac{X_{a}V}{{QX}_{a} - {QX}_{a}^{0}}$

Where

-   -   θ_(x)=Solids retention time in anaerobic digester (days)    -   X_(a)=concentration of methanogens    -   Q=influent and effluent flow rate    -   X_(a) ⁰=concentration of methanogens in the inlet stream, which        is normally considered zero for conventional activated sludge        process.

If about 50% of methanogens survive in the short solid retention timecontact stabilization process and are recycled back to anaerobicdigester, the solids retention time of the anaerobic digester could bedoubled, or the size of the anaerobic digester decreased by half. Forexample, in previously known systems a hydraulic retention time in ananaerobic digester was in many instances set at between about 20 andabout 30 days. With a treatment system operating in accordance someembodiments of the present application, this hydraulic retention timemay be reduced by about 50% to between about 10 and about 15 days.

In some embodiments of the apparatus and methods disclosed herein, ahydraulic retention time in a treatment system contact stabilizationvessel may be about one hour or less. A significant portion ofmethanogens can be recycled in the short solid retention time contactstabilization aerobic process, which can reduce the capital cost andoperational cost of the anaerobic digester(s). For example, the tankvolume of the anaerobic digester(s) could be decreased to bring thesafety factor to a range closer to those anaerobic digester(s) without amethanogen recycle process. With smaller volume, the capital cost of theanaerobic digesters and the mixing energy consumption of the anaerobicdigestion process would both decrease, which will make apparatus andprocesses in accordance with the present disclosure more cost effectivethan previously known apparatus and processes.

In other embodiments, the seeding of the anaerobic digester withrecycled methanogenic bacteria may provide for decreasing the hydraulicresidence time of sludge treated in the digester. This would result in adecreased cycle time, and thus an increased treatment capacity of thetreatment system. Increasing the amount of methanogens recycled to theanaerobic digester, by, for example, increasing an amount ofmethanogen-containing sludge directed into the digester, would providegreater opportunity to decrease the hydraulic residence time in thedigester and increase the treatment capacity of the system.

If a significant portion of methanogens can be recycled in the aerobiccontact stabilization process, the capital cost and operational cost ofthe anaerobic digesters could be decreased. For example, the tank volumeof the anaerobic digesters could be decreased to bring the safety factorto a range closer to those anaerobic digesters in systems not includinga methanogen recycle process. With smaller volume, the capital cost ofthe anaerobic digesters and the mixing energy consumption of theanaerobic digesters will both decrease, which will make the wastewatertreatment process more cost effective.

In certain embodiments, the contact tank is constantly seeded withnitrification bacteria (such as ammonia oxidizing and nitrite oxidizingbiomass) which can survive the anaerobic digester and which can berecycled back to the aerobic environment. For example, nitrification andde-nitrification can take place in the contact tank. Nitrification maybe carried out by two groups of slow-growing autotrophs:ammonium-oxidizing bacteria (AOB), which convert ammonia to nitrite, andnitrite-oxidizing bacteria (NOB), which oxidize nitrite to nitrate. Bothare slow growers and strict aerobes. In some embodiments of treatmentsystems disclosed herein, the nitrification bacteria are introduced toand/or grown in a contact tank, where they are captured in the floc.Some of the nitrification bacteria will pass out from the contact tankand be sent to an anaerobic digester.

It was previously believed that the strictly anaerobic conditions of theanaerobic digester would kill the nitrification bacteria. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of nitrification organisms in anaerobic andanoxic conditions that may occur in some biological nutrient removalprocesses. Nitrification bacteria which survive the anaerobic digesterand are returned to the aerobic part of the treatment process mayenhance the nitrification process performance in ways that can lowercapital costs, for example by providing for a reduced aerobic treatmentvessel size and/or reduced aerobic treatment hydraulic retention timeand/or an increased safety factor that would render the nitrificationprocess more stable in response to disruptions to the treatment process.Disruptions to the treatment process encompass deviations from desiredoperating parameters which may be caused by, for example, interruptionsin flow of material through the treatment system or a loss oftemperature control at one or more unit operations. The survival rate ofnitrification bacteria in an anaerobic digester could be increased bydecreasing a hydraulic residence time in the anaerobic digester, whichwould be accomplished if the anaerobic digester were seeded withrecycled methanogens, as described above.

A wastewater treatment system, indicated generally at 400 in FIG. 4,includes an anaerobic treatment unit 490, referred to herein as ananaerobic digester. The wastewater treatment system of FIG. 4 includes acontact tank 410, a DAF unit 420, a stabilization tank 430, a clarifier440, and associated fluid conduits 414, 424, 435, 445, 455, 465, and 475which are similar in structure and function to the contact tank 110, DAFunit 120, biological treatment unit 130, clarifier 140, and associatedfluid conduits 114, 124, 135, 145, 155, 165, and 175 of the systemillustrated in FIG. 1 and described above. A singular DAF unit 420 isillustrated in FIG. 4, although in alternate embodiments the treatmentsystem may use multiple DAF units as described above with reference tothe treatment system of FIG. 1.

In the system of FIG. 4, wastewater from a source of wastewater 405 isdirected into a primary clarifier 412 through an inlet of the primaryclarifier. A solids-rich fluid stream from the clarifier is directedthrough conduit 404 into an inlet of a thickener 480, which maycomprise, for example, a gravity belt thickener. A solids-lean effluentfrom the primary clarifier 412 is directed into an inlet of the contacttank 410 through conduit 402. A solids-rich output stream from thethickener 480 is directed to an inlet of the anaerobic digester 490through conduit 484. A solids-lean effluent from the thickener isdirected to an inlet of the contact tank 410 through conduit 482. Theanaerobic digester is also supplied with suspended solids removed frommixed liquor in the DAF unit 420 through conduits 425 and 484.

In some embodiments, a portion of the suspended solids removed from themixed liquor in the DAF unit 420 may be recycled to the contact tank 410through conduits 425 and 426 in a similar manner as the recycle ofsuspended solids removed in the DAF unit(s) 120 to the contact tank 110described above with reference to FIG. 1.

The solids-rich output stream from the thickener 480 and any suspendedsolids from the DAF unit 420 introduced into the anaerobic digester 490are combined and anaerobically digested in the anaerobic digester. Theanaerobic digestion process can be operated at temperatures betweenabout 20° C. and about 75° C., depending on the types of bacteriautilized during digestion. For example, use of mesophilic bacteriatypically requires operating temperatures of between about 20° C. andabout 45° C., while thermophilic bacteria typically require operatingtemperatures of between about 50° C. and about 75° C. In certainembodiments, the operating temperature may be between about 25° C. andabout 35° C. to promote mesophilic activity rather than thermophilicactivity. Depending on the other operating parameters, the retentiontime in the anaerobic digester can be between about seven and about 50days retention time, and in some embodiments, between about 15 and about30 days retention time. In certain embodiments, anaerobic digestion ofmixed liquor in the anaerobic digester may result in a reduction inoxygen demand of the mixed liquor of about 50%.

A first portion of an anaerobically digested sludge produced in theanaerobic digester may be recycled through an outlet of the anaerobicdigester and into the stabilization tank 430 through conduit 492. Thisrecycle stream may facilitate retaining sufficient solids in the systemto provide a desired residence time in the stabilization tank. Theanaerobically digested sludge recycled to the stabilization tank mayalso seed the stabilization tank with nitrification bacteria to enhancethe nitrification activity within the stabilization tank as describedabove. The anaerobically digested sludge recycled into the stabilizationtank may also contain methanogenic bacteria which are subsequentlyreturned to the anaerobic digester to enhance the performance of theanaerobic digester as described above.

In embodiments where the stabilization tank 430 includes an aeratedanoxic region and an aerobic region, such as in the biological treatmentunit 130 of FIG. 2 described above, the portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank. A second portion ofthe anaerobically digested sludge produced in the anaerobic digester maybe sent out of the system as waste solids through a conduit 495. Thefirst portion of the anaerobically digested sludge recycled into thestabilization tank 430 may be any amount between about 0% and about 100%of the anaerobically digested sludge produced in and output from theanaerobic digester, with the second portion, making up the balance, sentout of the system as waste solids through conduit 495. In someembodiments, between about 0% and about 80% of the anaerobicallydigested sludge is recycled from one or more outlets of the anaerobicdigester to one or more other unit operations of the treatment system.

In another embodiment of the wastewater treatment system, indicatedgenerally at 500 in FIG. 5, the first portion of the anaerobicallydigested sludge produced in the anaerobic digester is recycled throughan outlet of the anaerobic digester and into the inlet of the contacttank 410 through conduit 494, rather than into the stabilization tank430. This recycle stream may facilitate providing sufficient activatedsludge in the contact tank to absorb/absorb or enmesh BOD present in theinfluent wastewater. The anaerobically digested sludge recycled to thecontact tank may also seed the contact tank with nitrification bacteriato enhance the nitrification activity within the contact tank asdescribed above. The anaerobically digested sludge recycled into thecontact tank may also contain methanogenic bacteria which aresubsequently returned to the anaerobic digester to enhance theperformance of the anaerobic digester as described above. The firstportion of the anaerobically digested sludge recycled into the contacttank 410 may be any amount between about 0% and about 100% of theanaerobically digested sludge produced in and output from the anaerobicdigester, with a second portion, making up the balance, sent out of thesystem as waste solids through conduit 495.

In another embodiment of the wastewater treatment system, indicatedgenerally at 600 in FIG. 6, a first portion of the anaerobicallydigested sludge produced in the anaerobic digester may be recycledthrough an outlet of the anaerobic digester and into the inlet of thecontact tank 410 through conduit 494, and a second portion of theanaerobically digested sludge may be recycled through an outlet of theanaerobic digester and into the stabilization tank 430 through conduit492. These recycle streams may provide the benefits described above withregard to systems 400 and 500. A third portion of the anaerobicallydigested sludge may be directed to waste through conduit 495. The sum ofthe first portion of the anaerobically digested sludge and the secondportion of the anaerobic sludge may be any amount between about 0% andabout 100% of the anaerobically digested sludge produced in and outputfrom the anaerobic digester, with the third portion, making up thebalance, sent out of the system as waste solids through conduit 495. Therecycled anaerobic sludge may be split in any desired ratio between thefirst portion and the second portion. The first potion may comprise fromabout 0% to about 100% of all the anaerobically digested sludge producedin and output from the anaerobic digester with the sum of the secondportion and the third portion making up the balance.

Another embodiment of the wastewater treatment system, indicatedgenerally at 700 in FIG. 7, is similar to that illustrated in FIG. 6,however the thickener 480 is not utilized. Rather, the solids-rich fluidstream from the clarifier is directed through conduit 406 into an inletof the DAF unit 420. The DAF unit 420 of the system illustrated in FIG.7 performs the function of the thickener 480 of the system illustratedin FIG. 6. The utilization of the DAF unit 420 to perform the functionof the thickener may reduce or eliminate the need for a thickener in thesystem, which may reduce both capital and operational costs of thesystem. A first portion of the anaerobically digested sludge created inthe anaerobic digester 490 is recycled to the contact tank 410 and asecond portion is recycled to the stabilization tank 430 to provide thebenefits described above. A third portion of the anaerobically digestedsludge is directed to waste through conduit 495.

Further embodiments may include any combination of features of thesystems described above. For example, in some embodiments, a firstportion of the solids-rich fluid stream from the clarifier is directedthrough conduit 406 into an inlet of the DAF unit 420, while a secondportion is directed into a thickener 480. In any of the aboveembodiments, the stabilization tank 430 may include an aerated anoxicregion and an aerobic region. A first portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank and a second portionmay be recycled to the aerobic region. The ratio the amount of recycledanaerobic sludge directed to the aerated anoxic region to the amount ofrecycled anaerobic sludge directed to the aerobic region may be anyratio desired. Any of the above embodiments may include multiples of anyof the treatment units and/or conduits illustrated.

EXAMPLES Example 1

A wastewater treatment system 1000 was configured as illustrated in FIG.10, where the indicated unit operations and conduits have the samestructure and function as the identically indicated unit operations andconduits in FIGS. 4-7. The wastewater treatment system 1000 was used toexamine the effects of recycling removed solids from the DAF unit 420 tothe contact tank 410. By gradually increasing the amount of removedsolids from the DAF unit 420 recycled to the contact tank 410 from 0% ofthe solids removed in the DAF unit to about 90% of the solids removed inthe DAF unit over the course of three weeks, the suspended solids (MLSS)content of contact tank was brought up from 600 mg/L to over 1200 mg/L.The DAF dissolved solids content increased from 3%-4% prior to beginningthe recycle of solids from the DAF unit to the contact tank to above 5%after beginning the recycle of solids from the DAF unit to the contacttank. The total suspended solids (TSS) removal efficiency of the DAFunit increased from about 75% to over 85%. The COD removal of the DAFunit increased from about 70% to about 80% over the course of thetesting. These results are illustrated in the charts of FIG. 11 and FIG.12.

These results show that recycling removed solids from a DAF unit to acontact tank in a system such as that illustrated in FIG. 10 may providefor a greater amount of suspended solids in the contact tank. Theincreased amount of suspended solids in the contact tank increases theamount of suspended and soluble COD and BOD which may be removed fromwastewater influent to the contact tank and absorbed/adsorbed/enmeshedin the suspended solids and/or which may be oxidized in the contacttank. Recycling removed solids from a DAF unit to a contact tank in asystem such as that illustrated in FIG. 10 increases the efficiency ofthe removal of suspended solids in the DAF unit. These effects maydecrease the load on downstream unit operations and may reduce operatingcosts of the system as a whole and/or may reduce capital costs of thesystem by providing for smaller downstream processing units to beutilized. Further, a greater amount soluble BOD/COD from wastewaterinfluent to the system may be removed as solids in the DAF unit and maybe sent from the DAF unit to an anaerobic digester instead of an aerobictreatment unit operation, reducing the aeration power requirements ofthe system and increasing the amount of biogas that could be produced.

Prophetic Example 1

In this prophetic example, a water treatment system was configured asillustrated in FIG. 1 with the biological treatment unit 130 comprisinga single tank.

Assumptions of Feed:

The system was fed wastewater at a rate of 57,600 gallons/day (gpd), 40gallons per minute (gpm). The wastewater was assumed to be typical ofmunicipal wastewater, having a total BOD (tBOD) of 140 mg/l (67 lbs/day)of which 43% (60 mg/1, 29 lbs/day) was particulate (non-soluble) BOD(pBOD), and 57% (80 mg/1, 38 lbs/day) was soluble BOD (sBOD). Thewastewater was also assumed to include 100 mg/l (48 lbs/day) ofsuspended solids (SS), of which 19 lbs/day (48 lbs/day SS−29 lbs/daypBOD) was assumed to be inert (non-biological) material, and 6 lbs/dayof ammonia.

HDT Assumptions:

The hydraulic detention time (HDT) in the contact tank 110 was assumedto be 45 minutes and the hydraulic detention time (HDT) in thebiological treatment unit 130 was assumed to be five hours.

Flow Rate Through Contact Tank:

The ratio of return sludge sent from the clarifier 140 to the contacttank was set at 2.4 lb/lb of tBOD, for a (2.4)(67 lbs/day tBOD)=160lbs/day recycled sludge or 2,880 gpd (2.0 gpm), assuming a recycledsludge solids loading of 6,660 mg/l. The total flow through the contacttank was thus 57,600 gpd+2,880 gpd=60,480 gpd (42 gpm).

From laboratory bench scale testing, it was found that in the contacttank, approximately 50% of the sBOD was removed, with approximately ⅔ ofthe amount removed converted to SS, and approximately ⅓ of the amountremoved oxidized, for example, converted to carbon dioxide and water.Thus, it was assumed that in the contact tank 14 lbs/day of sBOD wasconverted to SS and 5 lbs/day of pBOD was oxidized. The total solidspassed through the contact tank was thus 160 lbs/day recycled sludge+48lbs/day suspended solids from influent wastewater+14 lbs/day sBODconverted to SS−5 lbs pBOD oxidized=217 lbs/day. The mixed liquorsuspended solids (MLSS) leaving the contact tank was thus ((217lbs/day)/(60,480 gpd))(453592.4 mg/lb)(0.2641721 gal/1)=430 mg/l.

The tBOD leaving the contact tank was 67 lbs/day input−5 lbs/dayoxidized=62 lbs/day (121 mg/l). The sBOD leaving the contact tank was 38lbs/day in−14 lbs/day converted to SS−5 lbs/day oxidized=19 lbs/day (37mg/l). The pBOD leaving the contact tank was 29 lbs/day influent+14lbs/day converted from sBOD=43 lbs/day (84 mg/l).

Flow Split into DAF and Biological Treatment Tank:

The flow out of the contact tank was split between the DAF units 120 andthe biological treatment unit 130. 46.5% (101 lbs/day, 28,080 gpd, 19.5gpm) of the output of the contact tank was directed to the DAF units and53.5% (116 lbs/day, 32,400 gpd, 22.5 gpm) was directed into thebiological treatment unit.

It was assumed that all recycled sludge directed to the DAF units (160lbs/day introduced into contact tank−116 lbs/day returned to biologicaltreatment tank=44 lbs/day) was removed in the DAF process.

BOD Influent to Biological Treatment Unit:

The total BOD to be treated in the biological treatment unit includesthe BOD from the contact tank (53.5% of 62 lbs/day=33 lbs/day) in 32,400gpd of influent plus BOD from the DAF units. The pBOD influent to theDAF units was 46.5% of 43 lbs/day output from contact tank=20 lbs/day.The sBOD influent to the DAF units was 46.5% of 19 lbs/day output fromthe contact tank=9 lbs/day at a flow rate of 28,800 gpd. Assuming 80% ofthe pBOD was removed in the DAF units, the tBOD flowing from the DAFunits to the biological treatment tank was (0.2*20 lbs/day pBOD)+9lbs/day sBOD=13 lbs/day tBOD. Thus the total influent BOD to thebiological treatment tank was 33 lbs/day from the contact tank+13lbs/day from the DAF units=46 lbs/day.

Solids in Biological Treatment Tank:

The biological treatment unit was sized to accommodate a BOD loading of29 lbs/1000 ft³, a common loading in the industry. This meant that thevolume of the biological treatment unit was (46 lbs/day influenttBOD)/(29 lbs/1000 ft³ tBOD loading)=1,600 ft³ (12,000 gal). This volumeresulted in a HDT in the biological treatment unit of (12,000 gal/57,600gpd)(24 hr/day)=5 hours. The total solids in the biological treatmentunit was set at 220 lbs, for a total MLSS of (220 lbs/12,000 gal)(0.264gal/l)(453,592 mg/lb)=2200 mg/l. Assuming a sludge yield of 95% of theBOD results in an amount of waste sludge produced in the biologicaltreatment unit of (0.95)(46 lbs/day tBOD)=44 lbs/day waste sludge. Thewaste sludge age would thus be (220 lbs total solids)/(44 lbs/day wastesludge)=5.2 days.

Biological Treatment Tank Oxygen Requirements:

It was assumed that 0.98 lbs of oxygen were required to oxidize a poundof BOD and 4.6 lbs of oxygen were required to oxidize a pound ofammonia. The oxygen requirement of the biological treatment unit wasthus (0.98 lbs O₂/lb BOD)(46 lbs tBOD/day)+(4.6 lbs O₂/lb ammonia)(6lb/day ammonia)=72.6 lb/day O₂ (3 lb O₂/hr). Using a FCF (FieldCorrection Factor−a correction factor to compensate for the reducedoxygen absorbing ability of mixed sludge in the biological treatmenttank as opposed to clean water) of 0.5, this results in a specificoxygen utilization rate (SOUR) of 6 lbs O₂/hr. Assuming diffused air wassupplied to the biological treatment tank from a aeration systemsubmerged by nine feet and a 6% oxygen transfer capability (OTE), thebiological treatment unit would require a flow of (6 lbsO₂/hr)(1/0.06)(1/60hour/min)(1/1.429 l/g O₂)(453.6 g/lb)(0.035ft³/l)=18.5 ft³/min (scfm), or if aerating with air with approximately20% O₂, 92.6 scfm.

Clarifier:

The clarifier was assumed to have a 61 ft² volume. 57,600 gpd flowedinto the clarifier, resulting in an overflow of 57,600 gpd/61 ft²=944gallon per ft² per day (gpsfd) overflow rate. Assuming an MLSS of 2200mg/l from the biological treatment tank and targeting a recycled sludge(RAS) concentration of 6600 mg/l and 50% of overflow recycled as RASgives a RAS flow rate of 20 gpm (28,800 gpd). It was assumed that 18 gpmRAS was recycled to the biological treatment tank and 2 gpm to thecontact tank. The solids loading of the clarifier was thus (57,600 gpdinfluent wastewater+28,800 gpd RAS)(2200 mg/l MLSS)(1/453592.4lb/mg)(3.79 l/gal)/(61 ft²)=(1588 lbs/day)/(61 ft²)=26 lb/ft²·day.

Solids Wasted:

Solids wasted in DAF units: 101 lbs/day (assuming 100% efficiency).

Ratio of sludge wasted to BOD treated: (101 lbs/day)/(67 lbs/day tBOD inwastewater influent)=1.5

With the addition of the DAF units to the treatment system in the aboveexample, the amount of tBOD to be treated in the biological treatmenttank was reduced from 62 lbs/day to 46 lbs/day, a reduction of 26%. Thisprovided for a reduced required size for the biological treatment tankto obtain a desired solids loading and resulted in a decrease in therequired amount of air needed to treat this tBOD in the biologicaltreatment tank. This would translate into a cost savings for bothcapital costs, for a reduced size of the biological treatment tank andaeration system, as well as a decreased operating cost due to thereduced amount of aeration required.

Prophetic Example 2

A simulation was performed using BIOWIN™ simulation software (EnviroSimAssociates Ltd., Ontario, Canada) to compare the performance of awastewater treatment system in accordance with an embodiment of thepresent invention with and without an anaerobic sludge recycle.

The wastewater treatment system without the anaerobic sludge recycleincluded was configured as illustrated in FIG. 8, indicated generally at800. This system is similar to that illustrated in FIG. 4, but with noanaerobic sludge recycle conduit 492 and with the addition of a membranebioreactor (MBR) 510 which receives a solids lean effluent from theclarifier 440 through conduit 442. The MBR produces a product waterpermeate which is removed from the system through conduit 445, and asolids-rich retentate, which is recycled to the DAF unit 480 throughconduit 444. The MBR 510 was simulated to perform complete nitrificationof the solids lean effluent from the clarifier 440.

The performance of the wastewater treatment of FIG. 8 was simulated andcompared to the simulated performance of the wastewater treatment system900 of FIG. 9. Wastewater treatment system 900 of FIG. 9 is similar towastewater treatment system 800 of FIG. 8, but with the addition of ananaerobic sludge recycle conduit 492 recycling anaerobically digestedsludge from the anaerobic digester 490 to the stabilization tank 430though conduit 492. In the simulation of the wastewater treatment system900, 45% of the anaerobically digested sludge output from the anaerobicdigester 490 was recycled to the stabilization tank 430, and 55% of theanaerobically digested sludge output from the anaerobic digester 490 wassent to waste.

The simulation of the performance of both systems 800 and 900 assumed aninfluent wastewater flow rate of 100 MGD. The influent wastewater wasassumed to have a COD of 500 mg/L, a total suspended solids (TSS) of 240mg/L, a Total Kjeldahl Nitrogen (TKN) of 40 mg/L, and a temperature of15° C.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in a decrease in the totaloxygen requirement for treating the influent wastewater as compared tothe system 800 of from 113,891 kg O₂/day to 102,724 kg O₂/day, a savingsof about 10%. Assuming an oxygen transfer energy requirement of 1.5 kgO₂/kwh, this reduction in oxygen consumption would reduce the powerrequirements associated with providing the oxygen from 75,988 kwh/day to68,483 kwh/day, a savings of 7,515 kwh/day.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in an increase in the amountof methane produced as compared to the system 800 from 1,348 scfm to1,652 scfm, an increase of about 23%. Assuming that 35% of the methanechemical energy could be converted to electricity, the potentialelectricity generation from the methane produced would increase from104,511 kwh/day to 128,989 kwh/day.

Combining the energy reduction from the reduced oxygen requirement withthe energy gain from the increased methane production results in anenergy savings of about 31,982 kwh/day for the system 900 including theanaerobically digested sludge recycle as compared to the system 800without the anaerobically digested sludge recycle.

The results of the simulation also indicated that adding theanaerobically digested sludge recycle of the system 900 to the system800 resulted in a reduction in biomass (sludge) production from 81,003pounds per day to 61,167 pounds per day, a reduction of about 25%.

This simulation data indicates that the addition of an anaerobicallydigested sludge recycle to wastewater treatment systems in accordancewith the present invention may result in a significant reduction inpower consumption and a significant decrease in waste sludge production,both of which would result in cost savings and enhancedenvironmental-friendliness of the wastewater treatment system.

Prophetic Example 3

Calculations were performed to compare the performance of a wastewatertreatment system in accordance with an embodiment of the presentinvention with and without a recycle of solids removed in a DAF unit ofthe system to a contact tank of the system. The wastewater treatmentsystem was configured as illustrated in FIG. 10.

It was assumed that the system was provided with 40 million gallons perday of wastewater influent with a BOD level of 250 mg/L (83,400 lbs/day)and suspended solids of 252 mg/L (84,000 lbs/day).

It was assumed that the biological treatment tank 430 operated with asolids retention time (SRT) of 5 days, a mixed liquor suspended solids(MLSS) concentration of 3,000 mg/L and a BOD loading of 45 lbs/1,000cubic feet (20.4 kg/28.3 cubic meters) and that all solids separated inthe clarifier 440 were recycled to the contact tank 410. The hydraulicdetention time (HDT) of the contact tank 410 was assumed to be 25minutes for the system operating without the DAF to contact tank solidsrecycle and one hour for the system operating with the DAF to contacttank solids recycle. The increase in HDT in the contact tank for thesystem when operating with the DAF to contact tank solids recycle was toprovide for the increased MLSS in the contact tank to adsorb additionalsoluble BOD in the contact tank as compared to the system operatingwithout the DAF to contact tank solids recycle. For the system operatingwith a recycle of solids from the DAF unit to the contact tank, it wasassumed that the DAF unit removed 308,000 lbs/day (139,706 kg/day) ofsolids from the mixed liquor passing through it and recycled 190,000lbs/day (86,183 kg/day, 62% of the solids removed) to the contact tankwhile directing 118,000 lbs/day (53,524 kg/day) of solids to theanaerobic digester 490.

A comparison of the results of the calculations comparing the systemwith and without the DAF to contact tank solids recycle is illustratedin Table 1 below:

TABLE 1 System operated System with without DAF → 62% DAF → Contact tankContact tank Parameter recycle recycle BOD treated in biological 41,20020,600 treatment tank (lbs/day) (18,688 kg/day)  (9,344 kg/day) Aerationenergy (both 600 410 contact tank and biological treatment tank, kW)Solids to anaerobic digester 103,000 115,000 (lbs/day) (46,720 kg/day)(52,163 kg/day) Solids destroyed (lbs/day) 43,900 55,900 (19,913 kg/day)(25,356 kg/day) Biogas produced (mcfd/day) 0.66 (18,633 0.84 (23,730cubic meters/day) cubic meters/day) Biogas energy (assuming 1,880 2,39040% conversion efficiency, kW) Net energy gain (kW) 1,280 1,880

These results show that providing a wastewater treatment system asconfigured in FIG. 10 with a recycle of solids removed in a DAF unit toa contact tank can significantly reduce the energy required to operatethe system as compared to an equivalent system without the recycle ofsolids from the DAF unit to the contact tank. Adding the DAF to contacttank solids recycle results in less BOD being sent for treatment in thebiological treatment tank (a reduction of (41,200−20,600)/41,200=50% inthe present example) which lowers the need for aeration in thebiological contact tank. A greater amount of biogas((0.84−0.66)/0.66=27% more in the present example) is produced whenadding the DAF to contact tank solids recycle to the system. Thecombined gain in biogas production and decrease in aeration energyrequirements results in a net energy gain of 1,880−1,280=600 kW whenadding the DAF to contact tank solids recycle to the system. At anestimated $0.10/kW energy cost, this net energy gain would yield a costsavings of about $530,000 per year.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is:
 1. A method of facilitating increased operatingefficiency of a wastewater treatment system, the method comprising:configuring a dissolved air flotation (DAF) unit in a wastewatertreatment system in fluid communication between a contact tank and abiological treatment unit to remove solids from a portion of a firstmixed liquor output from the contact tank prior to the portion of thefirst mixed liquor entering the biological treatment unit and to recycleat least a portion of the solids to the contact tank, the recycle of theat least a portion of the solids to the contact tank reducing an amountof biological oxygen demand to be treated in the biological treatmentunit as compared to the wastewater treatment system operating in theabsence of recycling the at least a portion of the solids to the contacttank, and to recycle the at least a portion of the solids from the DAFunit to the contact tank in an amount sufficient to reduce the energyconsumption of the wastewater treatment system.
 2. The method of claim1, wherein greater than 50% of the solids are recycled from the DAF unitto the contact tank.
 3. The method of claim 1, comprising recyclingsolids from the DAF unit to the contact tank in an amount sufficient toincrease biogas production of an anaerobic digester of the wastewatertreatment system having an inlet in fluid communication with an outletof the DAF unit, at least a second portion of the solids removed in theDAF unit being directed into the anaerobic digester.
 4. A wastewatertreatment system comprising: a contact tank having an first inletconfigured to receive wastewater to be treated, a second inlet, and anoutlet, the contact tank configured to mix the wastewater to be treatedwith activated sludge to form a first mixed liquor; a dissolved airflotation (DAF) unit having an inlet in fluid communication with theoutlet of the contact tank, a solids outlet, a DAF unit effluent outlet,and a gas inlet, the gas inlet configured to introduce gas into thedissolved air flotation unit to facilitate the flotation of suspendedmatter from the first mixed liquor and the removal of the suspendedmatter from the DAF unit, the solids outlet in fluid communication withthe first inlet of the contact tank and configured to transfer at leasta portion of the suspended matter from the DAF unit to the first inletof the contact tank; a biological treatment unit having a first inlet influid communication with the outlet of the contact tank, a second inlet,a third inlet in fluid communication with the DAF unit effluent outlet,and an outlet, the biological treatment unit configured to biologicallybreak down organic components of the first mixed liquor and of aneffluent from the DAF unit to form a second mixed liquor; and aclarifier having an inlet in fluid communication with the outlet of thebiological treatment unit, an effluent outlet, and a return activatedsludge outlet in fluid communication with the second inlet of thecontact tank and with the second inlet of the biological treatment unit,the clarifier configured to output a clarified effluent through theeffluent outlet and a return activated sludge though the returnactivated sludge outlet.
 5. The system of claim 4, where the biologicaltreatment unit includes: an aerated anoxic region having a first inletin fluid communication with the outlet of the contact tank, a secondinlet, and an outlet; and an aerobic region having a first inlet influid communication with the outlet of the aerated anoxic region, asecond inlet in fluid communication with the DAF unit effluent outlet,and an outlet.
 6. The system of claim 5, wherein the aerated anoxicregion and the aerobic region are included in a same treatment tank. 7.The system of claim 6, wherein the aerated anoxic region and the aerobicregion are separated by a partition.
 8. The system of claim 5, whereinthe aerated anoxic region is included in a first treatment tank and theaerobic region is included in a second treatment tank distinct from thefirst treatment tank.
 9. The system of claim 5, comprising a firstsub-system including the contact tank and the DAF unit which isphysically separated from a second sub-system including the biologicaltreatment unit and the clarifier.
 10. The system of claim 5, wherein thecontact tank and the aerated anoxic region are included in a same tank.11. The system of claim 4, further comprising an anaerobic digesterhaving an inlet in fluid communication with the second outlet of the DAFunit and an outlet.
 12. The system of claim 11, wherein the outlet ofthe anaerobic digester is in fluid communication with at least one ofthe contact tank and the biological treatment unit.
 13. The system ofclaim 12, further comprising a primary clarifier having an inlet influid communication with a source of the wastewater to be treated and asolids-lean outlet in fluid communication with the contact tank.
 14. Thesystem of claim 13, further comprising a thickener having an inlet influid communication with a solids-rich outlet of the primary clarifierand an outlet in fluid communication with the anaerobic digester. 15.The system of claim 13, wherein the primary clarifier further comprisesa solids-rich outlet in fluid communication with the DAF unit.
 16. Amethod of treating wastewater comprising: introducing the wastewaterinto a contact tank including an activated sludge; mixing the wastewaterwith activated sludge in the contact tank to form a mixed liquor;directing a first portion of the mixed liquor to a DAF unit; separatingthe first portion of the mixed liquor in the DAF unit to form a DAF uniteffluent and separated biosolids; directing at least a portion of theseparated biosolids from the DAF unit to the contact tank; directing asecond portion of the mixed liquor to a biological treatment unit;directing the DAF unit effluent to the biological treatment unit;biologically treating the mixed liquor and the DAF unit effluent in thebiological treatment unit to form a biologically treated mixed liquor;directing the biologically treated mixed liquor to a clarifier;separating the biologically treated mixed liquor in the clarifier toform a clarified effluent and a return activated sludge; recycling afirst portion of the return activated sludge to the contact tank;recycling a second portion of the return activated sludge to thebiological treatment unit; and directing the clarified effluent to atreated wastewater outlet.
 17. The method of claim 16, wherein thebiological treatment unit includes an aerated anoxic treatment unit andan aerobic treatment unit, and the method further comprises: directingthe second portion of the mixed liquor to the aerated anoxic treatmentunit; treating the second portion of the mixed liquor in the aeratedanoxic treatment unit to form an anoxic mixed liquor; directing theanoxic mixed liquor to the aerobic treatment unit; directing the DAFunit effluent to the aerobic treatment unit; treating the anoxic mixedliquor and the DAF unit effluent in the aerobic treatment tank to forman aerobic mixed liquor; directing the aerobic mixed liquor to theclarifier; separating the aerobic mixed liquor in the clarifier to formthe clarified effluent and the return activated sludge; and recyclingthe second portion of the return activated sludge to the aerated anoxictreatment unit.
 18. The method of claim 17, wherein the first portion ofthe return activated sludge and the second portion of the returnactivated sludge comprise about 100% of all return activated sludgeformed in the clarifier.
 19. The method of claim 18, wherein the firstportion of the return activated sludge comprises between about 10% andabout 20% of all return activated sludge recycled from the clarifier.20. The method of claim 16, wherein the first portion of the mixedliquor comprises between about one third and about two thirds of allmixed liquor formed in the contact tank.
 21. The method of claim 20,wherein the DAF unit removes between about 60% and about 100% ofsuspended solids in the first portion of the mixed liquor from the firstportion of the mixed liquor.
 22. The method of claim 21, wherein anamount of suspended solids removed in the DAF unit is adjusted basedupon a concentration of a bacteria in the biological treatment unit. 23.The method of claim 20, wherein the DAF unit removes between about 40%and about 80% of biological oxygen demand in the first portion of themixed liquor.
 24. The method of claim 16, further comprising treating atleast a portion of the waste biosolids in an anaerobic digester toproduce an anaerobically digested sludge.
 25. The method of claim 24,further comprising recycling at least a portion of the anaerobicallydigested sludge to at least one of the contact tank and the biologicaltreatment unit.
 26. The method of claim 25, further comprising:separating the water to be treated into a solids-lean portion and asolids-rich portion; directing the solids-rich portion into a thickenerto produce a solids-rich output and a solids-lean effluent; directingthe solids-lean portion into the contact tank; directing a solids-richoutput from the thickener into the anaerobic digester; and directing asolids-lean effluent of the thickener into the contact tank.
 27. Awastewater treatment system comprising: a contact tank having an firstinlet configured to receive wastewater to be treated, a second inlet,and an outlet, the contact tank configured to mix the wastewater to betreated with activated sludge to form a first mixed liquor; a dissolvedair flotation unit having an inlet in fluid communication with theoutlet of the contact tank and a gas inlet configured to introduce gasinto the dissolved air flotation unit to facilitate the flotation ofsuspended matter from the first mixed liquor and the removal of thesuspended matter from the dissolved air flotation unit, the dissolvedair flotation unit configured to separate the suspended matter from thefirst mixed liquor to form a solids-lean dissolved air flotation uniteffluent and waste solids, to output the waste solids through a solidsoutlet of the dissolved air flotation unit, and to output thesolids-lean dissolved air flotation unit effluent through an effluentoutlet of the dissolved air flotation unit; a biological treatment unithaving a return activated sludge inlet, a dissolved air flotationeffluent inlet in fluid communication with the effluent outlet of thedissolved air flotation unit, and an outlet, the biological treatmentunit configured to biologically break down organic components of thesolids-lean dissolved air flotation unit effluent to form a second mixedliquor; a clarifier having an inlet in fluid communication with theoutlet of the biological treatment unit, an effluent outlet, and areturn activated sludge outlet, the clarifier configured to output aclarified effluent through the effluent outlet and a return activatedsludge though the return activated sludge outlet; a dissolved airflotation unit effluent conduit extending between the effluent outlet ofthe dissolved air flotation unit and the dissolved air flotationeffluent inlet of the biological treatment unit; a first returnactivated sludge conduit extending between the return activated sludgeoutlet of the clarifier and the second inlet of the contact tank; and asecond return activated sludge conduit extending between the returnactivated sludge outlet of the clarifier and the return activated sludgeinlet of the biological treatment unit.
 28. The system of claim 27,wherein the solids outlet of the dissolved air flotation unit is influid communication with the first inlet of the contact tank and isconfigured to transfer at least a portion of the waste solids from thedissolved air flotation unit to the first inlet of the contact tank. 29.The system of claim 28, further comprising a mixed liquor conduitextending between the outlet of the contact tank and a mixed liquorinlet of the biological treatment unit, the mixed liquor conduitconfigured to direct a portion of the first mixed liquor from thecontact tank into the mixed liquor inlet of the biological treatmentunit.
 30. The system of claim 27, where the biological treatment unitincludes: an aerated anoxic region having an inlet and an outlet; and anaerobic region having a first inlet in fluid communication with theoutlet of the aerated anoxic region, a second inlet in fluidcommunication with the effluent outlet of the dissolved air flotationunit, and an outlet.
 31. The system of claim 30, wherein the aeratedanoxic region and the aerobic region are included in a same treatmenttank.
 32. The system of claim 31, wherein the aerated anoxic region andthe aerobic region are separated by a partition.
 33. The system of claim30, wherein the aerated anoxic region is included in a first treatmenttank and the aerobic region is included in a second treatment tankdistinct from the first treatment tank.
 34. The system of claim 30,comprising a first sub-system including the contact tank and thedissolved air flotation unit which is physically separated from a secondsub-system including the biological treatment unit and the clarifier.35. The system of claim 30, wherein the contact tank and the aeratedanoxic region are included in a same tank.
 36. The system of claim 27,further comprising an anaerobic digester having an inlet in fluidcommunication with the solids outlet of the dissolved air flotation unitand an outlet.
 37. The system of claim 26, wherein the outlet of theanaerobic digester is in fluid communication with at least one of thecontact tank and the biological treatment unit.
 38. The system of claim37, further comprising a primary clarifier having an inlet in fluidcommunication with a source of the wastewater to be treated and asolids-lean outlet in fluid communication with the contact tank.
 39. Thesystem of claim 38, further comprising a thickener having an inlet influid communication with a solids-rich outlet of the primary clarifierand an outlet in fluid communication with the anaerobic digester. 40.The system of claim 38, wherein the primary clarifier further comprisesa solids-rich outlet in fluid communication with the dissolved airflotation unit.
 41. A method of treating wastewater comprising:introducing the wastewater into a contact tank including an activatedsludge; mixing the wastewater with activated sludge in the contact tankto form a mixed liquor; directing the mixed liquor to a dissolved airflotation unit; separating the mixed liquor in the dissolved airflotation unit to form a solids-lean dissolved air flotation uniteffluent and waste biosolids; directing the solids-lean dissolved airflotation unit effluent to the biological treatment unit; biologicallytreating the solids-lean dissolved air flotation unit effluent in thebiological treatment unit to form a biologically treated mixed liquor;directing the biologically treated mixed liquor to a clarifier;separating the biologically treated mixed liquor in the clarifier toform a clarified effluent and a return activated sludge; recycling afirst portion of the return activated sludge to the contact tank;recycling a second portion of the return activated sludge to thebiological treatment unit; and directing the clarified effluent to atreated wastewater outlet.
 42. The method of claim 41, furthercomprising directing at least a portion of the waste biosolids from thedissolved air flotation unit to the contact tank.
 43. The method ofclaim 42, wherein directing the mixed liquor to the dissolved airflotation unit comprises directing a first portion of the mixed liquorto the dissolved air flotation unit and wherein the method furthercomprises directing a second portion of the mixed liquor from thecontact tank to the biological treatment unit.
 44. The method of claim41, wherein the biological treatment unit includes an aerated anoxictreatment unit and an aerobic treatment unit, and the method furthercomprises: directing the solids-lean dissolved air flotation tankeffluent to the aerobic treatment unit; treating the anoxic mixed liquorand the solids-lean dissolved air flotation unit effluent in the aerobictreatment tank to form an aerobic mixed liquor; directing the aerobicmixed liquor to the clarifier; separating the aerobic mixed liquor inthe clarifier to form the clarified effluent and the return activatedsludge; and recycling the second portion of the return activated sludgeto the aerated anoxic treatment unit.
 45. The method of claim 44,wherein the first portion of the return activated sludge and the secondportion of the return activated sludge comprise about 100% of all returnactivated sludge formed in the clarifier.
 46. The method of claim 44,wherein the first portion of the return activated sludge comprisesbetween about 10% and about 20% of all return activated sludge recycledfrom the clarifier.
 47. The method of claim 41, wherein the dissolvedair flotation unit removes between about 60% and about 100% of suspendedsolids in the mixed liquor as waste biosolids.
 48. The method of claim47, wherein an amount of suspended solids removed in the dissolved airflotation unit is adjusted based upon a concentration of a bacteria inthe biological treatment unit.
 49. The method of claim 41, wherein thedissolved air flotation unit removes between about 40% and about 80% ofbiological oxygen demand in the mixed liquor.
 50. The method of claim41, further comprising treating at least a portion of the wastebiosolids in an anaerobic digester to produce an anaerobically digestedsludge.
 51. The method of claim 50, further comprising recycling atleast a portion of the anaerobically digested sludge to at least one ofthe contact tank and the biological treatment unit.